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| author | Thomas Voss <mail@thomasvoss.com> | 2024-11-27 20:54:24 +0100 |
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| committer | Thomas Voss <mail@thomasvoss.com> | 2024-11-27 20:54:24 +0100 |
| commit | 4bfd864f10b68b71482b35c818559068ef8d5797 (patch) | |
| tree | e3989f47a7994642eb325063d46e8f08ffa681dc /doc/rfc/rfc1059.txt | |
| parent | ea76e11061bda059ae9f9ad130a9895cc85607db (diff) | |
doc: Add RFC documents
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diff --git a/doc/rfc/rfc1059.txt b/doc/rfc/rfc1059.txt new file mode 100644 index 0000000..8c134f6 --- /dev/null +++ b/doc/rfc/rfc1059.txt @@ -0,0 +1,3245 @@ +Network Working Group D. Mills +Request for Comments: 1059 University of Delaware + July 1988 + + Network Time Protocol (Version 1) + Specification and Implementation + +Status of this Memo + + This memo describes the Network Time Protocol (NTP), specifies its + formal structure and summarizes information useful for its + implementation. NTP provides the mechanisms to synchronize time and + coordinate time distribution in a large, diverse internet operating + at rates from mundane to lightwave. It uses a returnable-time design + in which a distributed subnet of time servers operating in a self- + organizing, hierarchical master-slave configuration synchronizes + logical clocks within the subnet and to national time standards via + wire or radio. The servers can also redistribute reference time via + local routing algorithms and time daemons. + + The NTP architectures, algorithms and protocols which have evolved + over several years of implementation and refinement are described in + this document. The prototype system, which has been in regular + operation in the Internet for the last two years, is described in an + Appendix along with performance data which shows that timekeeping + accuracy throughout most portions of the Internet can be ordinarily + maintained to within a few tens of milliseconds, even in cases of + failure or disruption of clocks, time servers or nets. This is a + Draft Standard for an Elective protocol. Distribution of this memo + is unlimited. + + Table of Contents + + 1. Introduction 3 + 1.1. Related Technology 4 + 2. System Architecture 6 + 2.1. Implementation Model 7 + 2.2. Network Configurations 9 + 2.3. Time Scales 10 + 3. Network Time Protocol 12 + 3.1. Data Formats 12 + 3.2. State Variables and Parameters 13 + 3.2.1. Common Variables 15 + 3.2.2. System Variables 17 + 3.2.3. Peer Variables 18 + 3.2.4. Packet Variables 19 + 3.2.5. Clock Filter Variables 19 + 3.2.6. Parameters 20 + + + +Mills [Page 1] + +RFC 1059 Network Time Protocol July 1988 + + + 3.3. Modes of Operation 21 + 3.4. Event Processing 22 + 3.4.1. Timeout Procedure 23 + 3.4.2. Receive Procedure 24 + 3.4.3. Update Procedure 27 + 3.4.4. Initialization Procedures 29 + 4. Filtering and Selection Algorithms 29 + 4.1. Clock Filter Algorithm 29 + 4.2 Clock Selection Algorithm 30 + 4.3. Variable-Rate Polling 32 + 5. Logical Clocks 33 + 5.1. Uniform Phase Adjustments 35 + 5.2. Nonuniform Phase Adjustments 36 + 5.3. Maintaining Date and Time 37 + 5.4. Calculating Estimates 37 + 6. References 40 + + Appendices + Appendix A. UDP Header Format 43 + Appendix B. NTP Data Format 44 + Appendix C. Timeteller Experiments 47 + Appendix D. Evaluation of Filtering Algorithms 49 + Appendix E. NTP Synchronization Networks 56 + + List of Figures + Figure 2.1. Implementation Model 8 + Figure 3.1. Calculating Delay and Offset 26 + Figure 5.1. Clock Registers 34 + Figure D.1. Calculating Delay and Offset 50 + Figure E.1. Primary Service Network 57 + + List of Tables + Table 2.1. Dates of Leap-Second Insertion 11 + Table 3.1. System Variables 14 + Table 3.2. Peer Variables 14 + Table 3.3. Packet Variables 15 + Table 3.4. Parameters 15 + Table 4.1. Outlyer Selection Procedure 32 + Table 5.1. Clock Parameters 35 + Table C.1. Distribution Functions 47 + Table D.1. Delay and Offset Measurements (UMD) 52 + Table D.2.a Delay and Offset Measurements (UDEL) 52 + Table D.2.b Offset Measurements (UDEL) 53 + Table D.3. Minimum Filter (UMD - NCAR) 54 + Table D.4. Median Filter (UMD - NCAR) 54 + Table D.5. Minimum Filter (UDEL - NCAR) 55 + Table E.1. Primary Servers 56 + + + + +Mills [Page 2] + +RFC 1059 Network Time Protocol July 1988 + + +1. Introduction + + This document describes the Network Time Protocol (NTP), including + the architectures, algorithms and protocols to synchronize local + clocks in a set of distributed clients and servers. The protocol was + first described in RFC-958 [24], but has evolved in significant ways + since publication of that document. NTP is built on the Internet + Protocol (IP) [10] and User Datagram Protocol (UDP) [6], which + provide a connectionless transport mechanism; however, it is readily + adaptable to other protocol suites. It is evolved from the Time + Protocol [13] and the ICMP Timestamp message [11], but is + specifically designed to maintain accuracy and robustness, even when + used over typical Internet paths involving multiple gateways and + unreliable nets. + + The service environment consists of the implementation model, service + model and time scale described in Section 2. The implementation + model is based on a multiple-process operating system architecture, + although other architectures could be used as well. The service + model is based on a returnable-time design which depends only on + measured offsets, or skews, but does not require reliable message + delivery. The subnet is a self-organizing, hierarchical master-slave + configuration, with synchronization paths determined by a minimum- + weight spanning tree. While multiple masters (primary servers) may + exist, there is no requirement for an election protocol. + + NTP itself is described in Section 3. It provides the protocol + mechanisms to synchronize time in principle to precisions in the + order of nanoseconds while preserving a non-ambiguous date well into + the next century. The protocol includes provisions to specify the + characteristics and estimate the error of the local clock and the + time server to which it may be synchronized. It also includes + provisions for operation with a number of mutually suspicious, + hierarchically distributed primary reference sources such as radio + clocks. + + Section 4 describes algorithms useful for deglitching and smoothing + clock-offset samples collected on a continuous basis. These + algorithms began with those suggested in [22], were refined as the + results of experiments described in [23] and further evolved under + typical operating conditions over the last two years. In addition, + as the result of experience in operating multiple-server nets + including radio-synchronized clocks at several sites in the US and + with clients in the US and Europe, reliable algorithms for selecting + good clocks from a population possibly including broken ones have + been developed and are described in Section 4. + + The accuracies achievable by NTP depend strongly on the precision of + + + +Mills [Page 3] + +RFC 1059 Network Time Protocol July 1988 + + + the local clock hardware and stringent control of device and process + latencies. Provisions must be included to adjust the software + logical clock time and frequency in response to corrections produced + by NTP. Section 5 describes a logical clock design evolved from the + Fuzzball implementation described in [15]. This design includes + offset-slewing, drift-compensation and deglitching mechanisms capable + of accuracies in order of a millisecond, even after extended periods + when synchronization to primary reference sources has been lost. + + The UDP and NTP packet formats are shown in Appendices A and B. + Appendix C presents the results of a survey of about 5500 Internet + hosts showing how their clocks compare with primary reference sources + using three different time protocols, including NTP. Appendix D + presents experimental results using several different deglitching and + smoothing algorithms. Appendix E describes the prototype NTP primary + service net, as well as proposed rules of engagement for its use. + +1.1. Related Technology + + Other mechanisms have been specified in the Internet protocol suite + to record and transmit the time at which an event takes place, + including the Daytime protocol [12], Time Protocol [13], ICMP + Timestamp message [11] and IP Timestamp option [9]. Experimental + results on measured times and roundtrip delays in the Internet are + discussed in [14], [23] and [31]. Other synchronization protocols + are discussed in [7], [17], [20] and [28]. NTP uses techniques + evolved from both linear and nonlinear synchronization methodology. + Linear methods used for digital telephone network synchronization are + summarized in [3], while nonlinear methods used for process + synchronization are summarized in [27]. + + The Fuzzball routing protocol [15], sometimes called Hellospeak, + incorporates time synchronization directly into the routing protocol + design. One or more processes synchronize to an external reference + source, such as a radio clock or NTP daemon, and the routing + algorithm constructs a minimum-weight spanning tree rooted on these + processes. The clock offsets are then distributed along the arcs of + the spanning tree to all processes in the system and the various + process clocks corrected using the procedure described in Section 5 + of this document. While it can be seen that the design of Hellospeak + strongly influenced the design of NTP, Hellospeak itself is not an + Internet protocol and is unsuited for use outside its local-net + environment. + + The Unix 4.3bsd model [20] uses a single master time daemon to + measure offsets of a number of slave hosts and send periodic + corrections to them. In this model the master is determined using an + election algorithm [25] designed to avoid situations where either no + + + +Mills [Page 4] + +RFC 1059 Network Time Protocol July 1988 + + + master is elected or more than one master is elected. The election + process requires a broadcast capability, which is not a ubiquitous + feature of the Internet. While this model has been extended to + support hierarchical configurations in which a slave on one network + serves as a master on the other [28], the model requires handcrafted + configuration tables in order to establish the hierarchy and avoid + loops. In addition to the burdensome, but presumably infrequent, + overheads of the election process, the offset measurement/correction + process requires twice as many messages as NTP per update. + + A good deal of research has gone into the issue of maintaining + accurate time in a community where some clocks cannot be trusted. A + truechimer is a clock that maintains timekeeping accuracy to a + previously published (and trusted) standard, while a falseticker is a + clock that does not. Determining whether a particular clock is a + truechimer or falseticker is an interesting abstract problem which + can be attacked using methods summarized in [19] and [27]. + + A convergence function operates upon the offsets between the clocks + in a system to increase the accuracy by reducing or eliminating + errors caused by falsetickers. There are two classes of convergence + functions, those involving interactive convergence algorithms and + those involving interactive consistency algorithms. Interactive + convergence algorithms use statistical clustering techniques such as + the fault-tolerant average algorithm of [17], the CNV algorithm of + [19], the majority-subset algorithm of [22], the egocentric algorithm + of [27] and the algorithms in Section 4 of this document. + + Interactive consistency algorithms are designed to detect faulty + clock processes which might indicate grossly inconsistent offsets in + successive readings or to different readers. These algorithms use an + agreement protocol involving successive rounds of readings, possibly + relayed and possibly augmented by digital signatures. Examples + include the fireworks algorithm of [17] and the optimum algorithm of + [30]. However, these algorithms require large numbers of messages, + especially when large numbers of clocks are involved, and are + designed to detect faults that have rarely been found in the Internet + experience. For these reasons they are not considered further in + this document. + + In practice it is not possible to determine the truechimers from the + falsetickers on other than a statistical basis, especially with + hierarchical configurations and a statistically noisy Internet. + Thus, the approach taken in this document and its predecessors + involves mutually coupled oscillators and maximum-likelihood + estimation and selection procedures. From the analytical point of + view, the system of distributed NTP peers operates as a set of + coupled phase-locked oscillators, with the update algorithm + + + +Mills [Page 5] + +RFC 1059 Network Time Protocol July 1988 + + + functioning as a phase detector and the logical clock as a voltage- + controlled oscillator. This similarity is not accidental, since + systems like this have been studied extensively [3], [4] and [5]. + + The particular choice of offset measurement and computation procedure + described in Section 3 is a variant of the returnable-time system + used in some digital telephone networks [3]. The clock filter and + selection algorithms are designed so that the clock synchronization + subnet self-organizes into a hierarchical master-slave configuration + [5]. What makes the NTP model unique is the adaptive configuration, + polling, filtering and selection functions which tailor the dynamics + of the system to fit the ubiquitous Internet environment. + +2. System Architecture + + The purpose of NTP is to connect a number of primary reference + sources, synchronized to national standards by wire or radio, to + widely accessible resources such as backbone gateways. These + gateways, acting as primary time servers, use NTP between them to + cross-check the clocks and mitigate errors due to equipment or + propagation failures. Some number of local-net hosts or gateways, + acting as secondary time servers, run NTP with one or more of the + primary servers. In order to reduce the protocol overhead the + secondary servers distribute time via NTP to the remaining local-net + hosts. In the interest of reliability, selected hosts can be + equipped with less accurate but less expensive radio clocks and used + for backup in case of failure of the primary and/or secondary servers + or communication paths between them. + + There is no provision for peer discovery, acquisition, or + authentication in NTP. Data integrity is provided by the IP and UDP + checksums. No circuit-management, duplicate-detection or + retransmission facilities are provided or necessary. The service can + operate in a symmetric mode, in which servers and clients are + indistinguishable, yet maintain a small amount of state information, + or in client/server mode, in which servers need maintain no state + other than that contained in the client request. A lightweight + association-management capability, including dynamic reachability and + variable polling rate mechanisms, is included only to manage the + state information and reduce resource requirements. Since only a + single NTP message format is used, the protocol is easily implemented + and can be used in a variety of solicited or unsolicited polling + mechanisms. + + It should be recognized that clock synchronization requires by its + nature long periods and multiple comparisons in order to maintain + accurate timekeeping. While only a few measurements are usually + adequate to reliably determine local time to within a second or so, + + + +Mills [Page 6] + +RFC 1059 Network Time Protocol July 1988 + + + periods of many hours and dozens of measurements are required to + resolve oscillator drift and maintain local time to the order of a + millisecond. Thus, the accuracy achieved is directly dependent on + the time taken to achieve it. Fortunately, the frequency of + measurements can be quite low and almost always non-intrusive to + normal net operations. + +2.1. Implementation Model + + In what may be the most common client/server model a client sends an + NTP message to one or more servers and processes the replies as + received. The server interchanges addresses and ports, overwrites + certain fields in the message, recalculates the checksum and returns + the message immediately. Information included in the NTP message + allows the client to determine the server time with respect to local + time and adjust the logical clock accordingly. In addition, the + message includes information to calculate the expected timekeeping + accuracy and reliability, thus select the best from possibly several + servers. + + While the client/server model may suffice for use on local nets + involving a public server and perhaps many workstation clients, the + full generality of NTP requires distributed participation of a number + of client/servers or peers arranged in a dynamically reconfigurable, + hierarchically distributed configuration. It also requires + sophisticated algorithms for association management, data + manipulation and logical clock control. Figure 2.1 shows a possible + implementation model including four processes sharing a partitioned + data base, with a partition dedicated to each peer and interconnected + by a message-passing system. + + + + + + + + + + + + + + + + + + + + + +Mills [Page 7] + +RFC 1059 Network Time Protocol July 1988 + + + +---------+ + | Update | + +--------->| +----------+ + | |Algorithm| | + | +----+----+ | + | | | + | V V + +----+----+ +---------+ +---------+ + | | | Local | | | + | Receive | | +---->| Timeout | + | | | Clock | | | + +---------+ +---------+ +-+-----+-+ + A A | | + | | V V + =========================================== + Peers Network Peers + + Figure 2.1. Implementation Model + + The timeout process, driven by independent timers for each peer, + collects information in the data base and sends NTP messages to other + peers in the net. Each message contains the local time the message + is sent, together with previously received information and other + information necessary to compute the estimated error and manage the + association. The message transmission rate is determined by the + accuracy expected of the local system, as well as its peers. + + The receive process receives NTP messages and perhaps messages in + other protocols as well, including ICMP, other UDP or TCP time + protocols, local-net protocols and directly connected radio clocks. + When an NTP message is received the offset between the sender clock + and the local clock is computed and incorporated into the data base + along with other information useful for error estimation and clock + selection. + + The update algorithm is initiated upon receipt of a message and at + other times. It processes the offset data from each peer and selects + the best peer using algorithms such as those described in Section 4. + This may involve many observations of a few clocks or a few + observations of many clocks, depending on the accuracies required. + + The local clock process operates upon the offset data produced by the + update algorithm and adjusts the phase and frequency of the logical + clock using mechanisms such as described in Section 5. This may + result in either a step change or a gradual slew adjustment of the + logical clock to reduce the offset to zero. The logical clock + provides a stable source of time information to other users of the + system and for subsequent reference by NTP itself. + + + +Mills [Page 8] + +RFC 1059 Network Time Protocol July 1988 + + +2.2. Network Configurations + + A primary time server is connected to a primary reference source, + usually a radio clock synchronized to national standard time. A + secondary time server derives time synchronization, possibly via + other secondary servers, from a primary server. Under normal + circumstances it is intended that a subnet of primary and secondary + servers assumes a hierarchical master-slave configuration with the + more accurate servers near the top and the less accurate below. + + Following conventions established by the telephone industry, the + accuracy of each server is defined by a number called its stratum, + with the stratum of a primary server assigned as one and each level + downwards in the hierarchy assigned as one greater than the preceding + level. With current technology and available receiving equipment, + single-sample accuracies in the order of a millisecond can be + achieved at the radio clock interface and in the order of a few + milliseconds at the packet interface to the net. Accuracies of this + order require special care in the design and implementation of the + operating system, such as described in [15], and the logical clock + mechanism, such as described in Section 5. + + As the stratum increases from one, the single-sample accuracies + achievable will degrade depending on the communication paths and + local clock stabilities. In order to avoid the tedious calculations + [4] necessary to estimate errors in each specific configuration, it + is useful to assume the errors accumulate approximately in proportion + to the minimum total roundtrip path delay between each server and the + primary reference source to which it is synchronized. This is called + the synchronization distance. + + Again drawing from the experience of the telephone industry, who + learned such lessons at considerable cost, the synchronization paths + should be organized to produce the highest accuracies, but must never + be allowed to form a loop. The clock filter and selection algorithms + used in NTP accomplish this by using a variant of the Bellman-Ford + distributed routing algorithm [29] to compute the minimum-weight + spanning trees rooted on the primary servers. This results in each + server operating at the lowest stratum and, in case of multiple peers + at the same stratum, at the lowest synchronization distance. + + As a result of the above design, the subnet reconfigures + automatically in a hierarchical master-slave configuration to produce + the most accurate time, even when one or more primary or secondary + servers or the communication paths between them fail. This includes + the case where all normal primary servers (e.g., backbone WWVB + clocks) on a possibly partitioned subnet fail, but one or more backup + primary servers (e.g., local WWV clocks) continue operation. + + + +Mills [Page 9] + +RFC 1059 Network Time Protocol July 1988 + + + However, should all primary servers throughout the subnet fail, the + remaining secondary servers will synchronize among themselves for + some time and then gradually drop off the subnet and coast using + their last offset and frequency computations. Since these + computations are expected to be very precise, especially in + frequency, even extend outage periods of a day or more should result + in timekeeping errors of not over a few tens of milliseconds. + + In the case of multiple primary servers, the spanning-tree + computation will usually select the server at minimum synchronization + distance. However, when these servers are at approximately the same + distance, the computation may result in random selections among them + as the result of normal dispersive delays. Ordinarily this does not + degrade accuracy as long as any discrepancy between the primary + servers is small compared to the synchronization distance. If not, + the filter and selection algorithms will select the best of the + available servers and cast out outlyers as intended. + +2.3. Time Scales + + Since 1972 the various national time scales have been based on + International Atomic Time (TA), which is currently maintained using + multiple cesium-beam clocks to an accuracy of a few parts in 10^12. + The Bureau International de l'Heure (BIH) uses astronomical + observations provided by the US Naval Observatory and other + observatories to determine corrections for small changes in the mean + rotation period of the Earth. This results in Universal Coordinated + Time (UTC), which is presently decreasing from TA at a fraction of a + second per year. When the magnitude of the correction approaches 0.7 + second, a leap second is inserted or deleted in the UTC time scale on + the last day of June or December. Further information on time scales + can be found in [26]. + + For the most precise coordination and timestamping of events since + 1972 it is necessary to know when leap seconds were inserted or + deleted in UTC and how the seconds are numbered. A leap second is + inserted following second 23:59:59 on the last day of June or + December and becomes second 23:59:60 of that day. A leap second + would be deleted by omitting second 23:59:59 on one of these days, + although this has never happened. Leap seconds were inserted on the + following fourteen occasions prior to January 1988 (courtesy US Naval + Observatory): + + + + + + + + + +Mills [Page 10] + +RFC 1059 Network Time Protocol July 1988 + + + 1 June 1972 8 December 1978 + 2 December 1972 9 December 1979 + 3 December 1973 10 June 1981 + 4 December 1974 11 June 1982 + 5 December 1975 12 June 1983 + 6 December 1976 13 June 1985 + 7 December 1977 14 December 1987 + + Table 2.1. Dates of Leap-Second Insertion + + Like UTC, NTP operates with an abstract oscillator synchronized in + frequency to the TA time scale. At 0000 hours on 1 January 1972 the + NTP time scale was set to 2,272,060,800, representing the number of + TA seconds since 0000 hours on 1 January 1900. The insertion of leap + seconds in UTC does not affect the oscillator itself, only the + translation between TA and UTC, or conventional civil time. However, + since the only institutional memory assumed by NTP is the UTC radio + broadcast service, the NTP time scale is in effect reset to UTC as + each offset estimate is computed. When a leap second is inserted in + UTC and subsequently in NTP, knowledge of all previous leap seconds + is lost. Thus, if a clock synchronized to NTP in early 1988 was used + to establish the time of an event that occured in early 1972, it + would be fourteen seconds early. + + When NTP is used to measure intervals between events that straddle a + leap second, special considerations apply. When it is necessary to + determine the elapsed time between events, such as the half life of a + proton, NTP timestamps of these events can be used directly. When it + is necessary to establish the order of events relative to UTC, such + as the order of funds transfers, NTP timestamps can also be used + directly; however, if it is necessary to establish the elapsed time + between events relative to UTC, such as the intervals between + payments on a mortgage, NTP timestamps must be converted to UTC using + the above table and its successors. + + The current formats used by NBS radio broadcast services [2] do not + include provisions for advance notice of leap seconds, so this + information must be determined from other sources. NTP includes + provisions to distribute advance warnings of leap seconds using the + Leap Indicator bits described in Section 3. The protocol is designed + so that these bits can be set manually at the primary clocks and then + automatically distributed throughout the system for delivery to all + logical clocks and then effected as described in Section 5. + + + + + + + + +Mills [Page 11] + +RFC 1059 Network Time Protocol July 1988 + + +3. Network Time Protocol + + This section consists of a formal definition of the Network Time + Protocol, including its data formats, entities, state variables, + events and event-processing procedures. The specification model is + based on the implementation model illustrated in Figure 2.1, but it + is not intended that this model is the only one upon which a + specification can be based. In particular, the specification is + intended to illustrate and clarify the intrinsic operations of NTP + and serve as a foundation for a more rigorous, comprehensive and + verifiable specification. + +3.1. Data Formats + + All mathematical operations expressed or implied herein are in + two's-complement arithmetic. Data are specified as integer or + fixed-point quantities. Since various implementations would be + expected to scale externally derived quantities for internal use, + neither the precision nor decimal-point placement for fixed-point + quantities is specified. Unless specified otherwise, all quantities + are unsigned and may occupy the full field width, if designated, with + an implied zero preceding the most significant (leftmost) bit. + Hardware and software packages designed to work with signed + quantities will thus yield surprising results when the most + significant (sign) bit is set. It is suggested that externally + derived, unsigned fixed-point quantities such as timestamps be + shifted right one bit for internal use, since the precision + represented by the full field width is seldom justified. + + Since NTP timestamps are cherished data and, in fact, represent the + main product of the protocol, a special timestamp format has been + established. NTP timestamps are represented as a 64-bit unsigned + fixed-point number, in seconds relative to 0000 UT on 1 January 1900. + The integer part is in the first 32 bits and the fraction part in the + last 32 bits, as shown in the following diagram. + + 0 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Integer Part | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Fraction Part | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + This format allows convenient multiple-precision arithmetic and + conversion to Time Protocol representation (seconds), but does + complicate the conversion to ICMP Timestamp message representation + (milliseconds). The precision of this representation is about 0.2 + + + +Mills [Page 12] + +RFC 1059 Network Time Protocol July 1988 + + + nanosecond, which should be adequate for even the most exotic + requirements. + + Timestamps are determined by copying the current value of the logical + clock to a timestamp variable when some significant event, such as + the arrival of a message, occurs. In order to maintain the highest + accuracy, it is important that this be done as close to the hardware + or software driver associated with the event as possible. In + particular, departure timestamps should be redetermined for each + link-level retransmission. In some cases a particular timestamp may + not be available, such as when the host is rebooted or the protocol + first starts up. In these cases the 64-bit field is set to zero, + indicating the value is invalid or undefined. + + Note that since some time in 1968 the most significant bit (bit 0 of + the Integer Part) has been set and that the 64-bit field will + overflow some time in 2036. Should NTP be in use in 2036, some + external means will be necessary to qualify time relative to 1900 and + time relative to 2036 (and other multiples of 136 years). + Timestamped data requiring such qualification will be so precious + that appropriate means should be readily available. There will exist + an 0.2-nanosecond interval, henceforth ignored, every 136 years when + the 64-bit field will be zero and thus considered invalid. + +3.2. State Variables and Parameters + + Following is a tabular summary of the various state variables and + parameters used by the protocol. They are separated into classes of + system variables, which relate to the operating system environment + and logical clock mechanism; peer variables, which are specific to + each peer operating in symmetric mode or client mode; packet + variables, which represent the contents of the NTP message; and + parameters, which are fixed in all implementations of the current + version. For each class the description of the variable is followed + by its name and the procedure or value which controls it. Note that + variables are in lower case, while parameters are in upper case. + + + + + + + + + + + + + + + +Mills [Page 13] + +RFC 1059 Network Time Protocol July 1988 + + + System Variables Name Control + ------------------------------------------------------- + Logical Clock sys.clock update + Clock Source sys.peer selection + algorithm + Leap Indicator sys.leap update + Stratum sys.stratum update + Precision sys.precision system + Synchronizing Distance sys.distance update + Estimated Drift Rate sys.drift system + Reference Clock Identifier sys.refid update + Reference Timestamp sys.reftime update + + Table 3.1. System Variables + + Peer Variables Name Control + ------------------------------------------------------- + Peer Address peer.srcadr system + Peer Port peer.srcport system + Local Address peer.dstadr system + Local Port peer.dstport system + Peer State peer.state receive, + transmit + Reachability Register peer.reach receive, + transmit + Peer Timer peer.timer system + Timer Threshold peer.threshold system + Leap Indicator peer.leap receive + Stratum peer.stratum receive + Peer Poll Interval peer.ppoll receive + Host Poll Interval peer.hpoll receive, + transmit + Precision peer.precision receive + Synchronizing Distance peer.distance receive + Estimated Drift Rate peer.drift receive + Reference Clock Identifier peer.refid receive + Reference Timestamp peer.reftime receive + Originate Timestamp peer.org receive + Receive Timestamp peer.rec receive + Filter Register peer.filter filter + algorithm + Delay Estimate peer.delay filter + algorithm + Offset Estimate peer.offset filter + algorithm + Dispersion Estimate peer.dispersion filter + + Table 3.2. Peer Variables + + + +Mills [Page 14] + +RFC 1059 Network Time Protocol July 1988 + + + Packet Variables Name Control + ------------------------------------------------------- + Peer Address pkt.srcadr transmit + Peer Port pkt.srcport transmit + Local Address pkt.dstadr transmit + Local Port pkt.dstport transmit + Leap Indicator pkt.leap transmit + Version Number pkt.version transmit + Stratum pkt.stratum transmit + Poll pkt.poll transmit + Precision pkt.precision transmit + Synchronizing Distance pkt.distance transmit + Estimated Drift Rate pkt.drift transmit + Reference Clock Identifier pkt.refid transmit + Reference Timestamp pkt.reftime transmit + Originate Timestamp pkt.org transmit + Receive Timestamp pkt.rec transmit + Transmit Timestamp pkt.xmt transmit + + Table 3.3. Packet Variables + + Parameters Name Value + ------------------------------------------------------- + NTP Version NTP.VERSION 1 + NTP Port NTP.PORT 123 + Minimum Polling Interval NTP.MINPOLL 6 (64 sec) + Maximum Polling Interval NTP.MAXPOLL 10 (1024 + sec) + Maximum Dispersion NTP.MAXDISP 65535 ms + Reachability Register Size PEER.WINDOW 8 + Shift Register Size PEER.SHIFT 4/8 + Dispersion Threshold PEER.THRESHOLD 500 ms + Filter Weight PEER.FILTER .5 + Select Weight PEER.SELECT .75 + + Table 3.4. Parameters + + Following is a description of the various variables used in the + protocol. Additional details on formats and use are presented in + later sections and appendices. + +3.2.1. Common Variables + + The following variables are common to the system, peer and packet + classes. + + Peer Address (peer.srcadr, pkt.srcadr) Peer Port (peer.srcport, + pkt.srcport) + + + +Mills [Page 15] + +RFC 1059 Network Time Protocol July 1988 + + + These are the 32-bit Internet address and 16-bit port number of + the remote host. + + Local Address (peer.dstadr, pkt.dstadr) Local Port (peer.dstport, + pkt.dstport) + + These are the 32-bit Internet address and 16-bit port number of + the local host. They are included among the state variables to + support multi-homing. + + Leap Indicator (sys.leap, peer.leap, pkt.leap) + + This is a two-bit code warning of an impending leap second to be + inserted in the NTP time scale. The bits are set before 23:59 on + the day of insertion and reset after 00:01 on the following day. + This causes the number of seconds (rollover interval) in the day + of insertion to be increased or decreased by one. In the case of + primary servers the bits are set by operator intervention, while + in the case of secondary servers the bits are set by the protocol. + The two bits are coded as follows: + + 00 no warning (day has 86400 seconds) + 01 +1 second (day has 86401 seconds) + seconds) + 10 -1 second (day has 86399 seconds) + seconds) + 11 alarm condition (clock not synchronized) + + In all except the alarm condition (11) NTP itself does nothing + with these bits, except pass them on to the time-conversion + routines that are not part of NTP. The alarm condition occurs + when, for whatever reason, the logical clock is not synchronized, + such as when first coming up or after an extended period when no + outside reference source is available. + + Stratum (sys.stratum, peer.stratum, pkt.stratum) + + This is an integer indicating the stratum of the logical clock. A + value of zero is interpreted as unspecified, one as a primary + clock (synchronized by outside means) and remaining values as the + stratum level (synchronized by NTP). For comparison purposes a + value of zero is considered greater than any other value. + + Peer Poll Interval (peer.ppoll, pkt.poll) + + This is a signed integer used only in symmetric mode and + indicating the minimum interval between messages sent to the peer, + in seconds as a power of two. For instance, a value of six + + + +Mills [Page 16] + +RFC 1059 Network Time Protocol July 1988 + + + indicates a minimum interval of 64 seconds. The value of this + variable must not be less than NTP.MINPOLL and must not be greater + than NTP.MAXPOLL. + + Precision (sys.precision, peer.precision, pkt.precision) + + This is a signed integer indicating the precision of the logical + clock, in seconds to the nearest power of two. For instance, a + 60-Hz line-frequency clock would be assigned the value -6, while a + 1000-Hz crystal-derived clock would be assigned the value -10. + + Synchronizing Distance (sys.distance, peer.distance, pkt.distance) + + This is a fixed-point number indicating the estimated roundtrip + delay to the primary clock, in seconds. + + + Estimated Drift Rate (sys.drift, peer.drift, pkt.drift) + + This is a fixed-point number indicating the estimated drift rate + of the local clock, in dimensionless units. + + Reference Clock Identifier (sys.refid, peer.refid, pkt.refid) + + This is a code identifying the particular reference clock or + server. The interpretation of the value depends on the stratum. + For stratum values of zero (unspecified) or one (primary clock), + the value is an ASCII string identifying the reason or clock, + respectively. For stratum values greater than one (synchronized + by NTP), the value is the 32-bit Internet address of the reference + server. + + Reference Timestamp (sys.reftime, peer.reftime, pkt.reftime) + + This is the local time, in timestamp format, when the logical + clock was last updated. If the logical clock has never been + synchronized, the value is zero. + +3.2.2. System Variables + + The following variables are used by the operating system in order to + synchronize the logical clock. + + Logical Clock (sys.clock) + + This is the current local time, in timestamp format. Local time + is derived from the hardware clock of the particular machine and + increments at intervals depending on the design used. An + + + +Mills [Page 17] + +RFC 1059 Network Time Protocol July 1988 + + + appropriate design, including slewing and drift-compensation + mechanisms, is described in Section 5. + + Clock Source (sys.peer) + + This is a selector identifying the current clock source. Usually + this will be a pointer to a structure containing the peer + variables. + +3.2.3. Peer Variables + + Following is a list of state variables used by the peer management + and measurement functions. There is one set of these variables for + every peer operating in client mode or symmetric mode. + + Peer State (peer.state) + + This is a bit-encoded quantity used for various control functions. + + Host Poll Interval (peer.hpoll) + + This is a signed integer used only in symmetric mode and + indicating the minimum interval between messages expected from the + peer, in seconds as a power of two. For instance, a value of six + indicates a minimum interval of 64 seconds. The value of this + variable must not be less than NTP.MINPOLL and must not be greater + than NTP.MAXPOLL. + + Reachability Register (peer.reach) + + This is a code used to determine the reachability status of the + peer. It is used as a shift register, with bits entering from the + least significant (rightmost) end. The size of this register is + specified as PEER.SHIFT bits. + + Peer Timer (peer.timer) + + This is an integer counter used to control the interval between + transmitted NTP messages. + + Timer Threshold (peer.threshold) + + This is the timer value which, when reached, causes the timeout + procedure to be executed. + + Originate Timestamp (peer.org, pkt.org) + + This is the local time, in timestamp format, at the peer when its + + + +Mills [Page 18] + +RFC 1059 Network Time Protocol July 1988 + + + latest NTP message was sent. If the peer becomes unreachable the + value is set to zero. + + Receive Timestamp (peer.rec, pkt.rec) + + This is the local time, in timestamp format, when the latest NTP + message from the peer arrived. If the peer becomes unreachable + the value is set to zero. + +3.2.4. Packet Variables + + Following is a list of variables used in NTP messages in addition to + the common variables above. + + Version Number (pkt.version) + + This is an integer indicating the version number of the sender. + NTP messages will always be sent with the current version number + NTP.VERSION and will always be accepted if the version number + matches NTP.VERSION. Exceptions may be advised on a case-by-case + basis at times when the version number is changed. + + Transmit Timestamp (pkt.xmt) + + This is the local time, in timestamp format, at which the NTP + message departed the sender. + +3.2.5. Clock Filter Variables + + When the filter and selection algorithms suggested in Section 4 are + used, the following state variables are defined. There is one set of + these variables for every peer operating in client mode or symmetric + mode. + + Filter Register (peer.filter) + + This is a shift register of PEER.WINDOW bits, where each stage is + a tuple consisting of the measured delay concatenated with the + measured offset associated with a single observation. + Delay/offset observations enter from the least significant + (rightmost) right and are shifted towards the most significant + (leftmost) end and eventually discarded as new observations + arrive. The register is cleared to zeros when (a) the peer + becomes unreachable or (b) the logical clock has just been reset + so as to cause a significant discontinuity in local time. + + + + + + +Mills [Page 19] + +RFC 1059 Network Time Protocol July 1988 + + + Delay Estimate (peer.delay) + + This is a signed, fixed-point number indicating the latest delay + estimate output from the filter, in seconds. While the number is + signed, only those values greater than zero represent valid delay + estimates. + + Offset Estimate (peer.offset) + + This is a signed, fixed-point number indicating the latest offset + estimate output from the filter, in seconds. + + Dispersion Estimate (peer.dispersion) + + This is a fixed-point number indicating the latest dispersion + estimate output from the filter, in scrambled units. + +3.2.6. Parameters + + Following is a list of parameters assumed for all implementations + operating in the Internet system. It is necessary to agree on the + values for these parameters in order to avoid unnecessary network + overheads and stable peer associations. + + Version Number (NTP.VERSION) + + This is the NTP version number, currently one (1). + + NTP Port (NTP.PORT) + + This is the port number (123) assigned by the Internet Number Czar + to NTP. + + Minimum Polling Interval (NTP.MINPOLL) + + This is the minimum polling interval allowed by any peer of the + Internet system, currently set to 6 (64 seconds). + + Maximum Polling Interval (NTP.MAXPOLL) + + This is the maximum polling interval allowed by any peer of the + Internet system, currently set to 10 (1024 seconds). + + Maximum Dispersion (NTP.MAXDISP) + + This is the maximum dispersion assumed by the filter algorithms, + currently set to 65535 milliseconds. + + + + +Mills [Page 20] + +RFC 1059 Network Time Protocol July 1988 + + + Reachability Register Size (PEER.WINDOW) + + This is the size of the Reachability Register (peer.reach), + currently set to eight (8) bits. + + Shift Register Size (PEER.SHIFT) + + When the filter and selection algorithms suggested in Section 4 + are used, this is the size of the Clock Filter (peer.filter) shift + register, in bits. For crystal-stabilized oscillators a value of + eight (8) is suggested, while for mains-frequency oscillators a + value of four (4) is suggested. Additional considerations are + given in Section 5. + + Dispersion Threshold (PEER.THRESHOLD) + + When the filter and selection algorithms suggested in Section 4 + are used, this is the threshold used to discard noisy data. While + a value of 500 milliseconds is suggested, the value may be changed + to suit local conditions on particular peer paths. + + Filter Weight (PEER.FILTER) + + When the filter algorithm suggested in Section 4 is used, this is + the filter weight used to discard noisy data. While a value of + 0.5 is suggested, the value may be changed to suit local + conditions on particular peer paths. + + Select Weight (PEER.SELECT) + + When the selection algorithm suggested in Section 4 is used, this + is the select weight used to discard outlyers. data. While a + value of 0.75 is suggested, the value may be changed to suit local + conditions on particular peer paths. + +3.3. Modes of Operation + + An NTP host can operate in three modes: client, server and + symmetric. The mode of operation is determined by whether the source + port (peer.srcport) or destination port (peer.dstport) peer variables + contain the assigned NTP service port number NTP.PORT (123) as shown + in the following table. + + + + + + + + + +Mills [Page 21] + +RFC 1059 Network Time Protocol July 1988 + + + peer.srcport peer.dstport Mode + ------------------------------------------- + not NTP.PORT not NTP.PORT not possible + not NTP.PORT NTP.PORT server + NTP.PORT not NTP.PORT client + NTP.PORT NTP.PORT symmetric + + A host operating in client mode occasionally sends an NTP message to + a host operating in server mode. The server responds by simply + interchanging addresses and ports, filling in the required + information and returning the message to the client. Servers then + need retain no state information between client requests. Clients + are free to manage the intervals between sending NTP messages to suit + local conditions. + + In symmetric mode the client/server distinction disappears. Each + host maintains a table with as many entries as active peers. Each + entry includes a code uniquely identifying the peer (e.g., Internet + address and port), together with status information and a copy of the + timestamps last received. A host operating in symmetric mode + periodically sends NTP messages to each peer including the latest + copy of the timestamps. The intervals between sending NTP messages + are managed jointly by the host and each peer using the polling + variables peer.ppoll and peer.hpoll. + + When a pair of peers operating in symmetric mode exchange NTP + messages and each determines that the other is reachable, an + association is formed. One or both peers must be in active state; + that is, sending messages to the other regardless of reachability + status. A peer not in active state is in passive state. If a peer + operating in passive state discovers that the other peer is no longer + reachable, it ceases sending messages and reclaims the storage and + timer resources used by the association. A peer operating in client + mode is always in active state, while a peer operating in server mode + is always in passive state. + +3.4. Event Processing + + The significant events of interest in NTP occur upon expiration of + the peer timer, one of which is dedicated to each peer operating in + symmetric or client modes, and upon arrival of an NTP message from + the various peers. An event can also occur as the result of an + operator command or detected system fault, such as a primary clock + failure. This section describes the procedures invoked when these + events occur. + + + + + + +Mills [Page 22] + +RFC 1059 Network Time Protocol July 1988 + + +3.4.1. Timeout Procedure + + The timeout procedure is called in client and symmetric modes when + the peer timer (peer.timer) reaches the value of the timer threshold + (peer.threshold) variable. First, the reachability register + (peer.reach) is shifted one position to the left and a zero replaces + the vacated bit. Then an NTP message is constructed and sent to the + peer. If operating in active state or in passive state and + peer.reach is nonzero (reachable), the peer.timer is reinitialized + (resumes counting from zero) and the value of peer.threshold is set + to: + + peer.threshold <- max( min( peer.ppoll, peer.hpoll, + NTP.MAXPOLL), NTP.MINPOLL) . + + If operating in active state and peer.reach is zero (unreachable), + the peer variables are updated as follows: + + peer.hpoll <- NTP.MINPOLL + peer.disp <- NTP.MAXDISP + peer.filter <- 0 (cleared) + peer.org <- 0 + peer.rec <- 0 + + Then the clock selection algorithm is called, which may result in a + new clock source (sys.peer). In other cases the protocol ceases + operation and the storage and timer resources are reclaimed for + subsequent use. + + An NTP message is constructed as follows (see Appendices A and B for + formats). First, the IP and UDP packet variables are copied from the + peer variables (note the interchange of source and destination + addresses and ports): + + pkt.srcadr <- peer.dstadr pkt.srcport <- peer.dstport + pkt.dstadr <- peer.srcadr pkt.dstport <- peer.srcport + + Next, the NTP packet variables are copied (rescaled as necessary) + from the system and peer variables: + + pkt.leap <- sys.leap pkt.distance <- sys.distance + pkt.version <- NTP.VERSION pkt.drift <- sys.drift + pkt.stratum <- sys.stratum pkt.refid <- sys.refid + pkt.poll <- peer.hpoll pkt.reftime <- sys.reftime + pkt.precision <- sys.precision + + Finally, the NTP packet timestamp variables are copied, depending on + whether the peer is operating in symmetric mode and reachable, in + + + +Mills [Page 23] + +RFC 1059 Network Time Protocol July 1988 + + + symmetric mode and not reachable (but active) or in client mode: + + Symmetric Reachable Symmetric Active Client + - ------------------- ------------------- ------------------- + pkt.org <- peer.org pkt.org <- 0 pkt.org <- sys.clock + pkt.rec <- peer.rec pkt.rec <- 0 pkt.rec <- sys.clock + pkt.xmt <- sys.clock pkt.xmt <- sys.clock pkt.xmt <- sys.clock + + Note that the order of copying should be designed so that the time to + perform the copy operations themselves does not degrade the + measurement accuracy, which implies that the sys.clock values should + be copied last. The reason for the choice of zeros to fill the + pkt.org and pkt.rec packet variables in the symmetric unreachable + case is to avoid the use of old data after a possibly extensive + period of unreachability. The reason for the choice of sys.clock to + fill these variables in the client case is that, if for some reason + the NTP message is returned by the recipient unaltered, as when + testing with an Internet-echo server, this convention still allows at + least the roundtrip time to be accurately determined without special + handling. + +3.4.2. Receive Procedure + + The receive procedure is executed upon arrival of an NTP message. If + the version number of the message (pkt.version) does not match the + current version number (NTP.VERSION), the message is discarded; + however, exceptions may be advised on a case-by-case basis at times + when the version number is changed. + + If the clock of the sender is unsynchronized (pkt.leap = 11), or the + receiver is in server mode or the receiver is in symmetric mode and + the stratum of the sender is greater than the stratum of the receiver + (pkt.stratum > sys.stratum), the message is simply returned to the + sender along with the timestamps. In this case the addresses and + ports are interchanged in the IP and UDP headers: + + pkt.srcadr <-> pkt.dstadr pkt.srcport <-> pkt.dstport + + The following packet variables are updated from the system variables: + + pkt.leap <- sys.leap pkt.distance <- sys.distance + pkt.version <- NTP.VERSION pkt.drift <- sys.drift + pkt.stratum <- sys.stratum pkt.refid <- sys.refid + pkt.precision <- sys.precision pkt.reftime <- sys.reftime + + Note that the pkt.poll packet variable is unchanged. The timestamps + are updated in the order shown: + + + + +Mills [Page 24] + +RFC 1059 Network Time Protocol July 1988 + + + pkt.org <- pkt.xmt + pkt.rec <- sys.clock + pkt.xmt <- sys.clock + + Finally, the message is forwarded to the sender and the server + receive procedure terminated at this point. + + If the above is not the case, the source and destination Internet + addresses and ports in the IP and UDP headers are matched to the + correct peer. If there is a match, processing continues at the next + step below. If there is no match and symmetric mode is not indicated + (either pkt.srcport or pkt.dstport not equal to NTP.PORT), the + message must be a reply to a previously sent message from a client + which is no longer in operation. In this case the message is dropped + and the receive procedure terminated at this point. + + If there is no match and symmetric mode is indicated, (both + pkt.srcport and pkt.dstport equal to NTP.PORT), an implementation- + specific instantiation procedure is called to create and initialize a + new set of peer variables and start the peer timer. The following + peer variables are set from the IP and UDP headers: + + peer.srcadr <- pkt.srcadr peer.srcport <- pkt.srcport + peer.dstadr <- pkt.dstadr peer.dstport <- pkt.dstport + + + The following peer variables are initialized: + + peer.state <- symmetric (passive) + peer.timer <- 0 (enabled) + peer.hpoll <- NTP.MINPOLL + peer.disp <- NTP.MAXDISP + + The remaining peer variables are undefined and set to zero. + + Assuming that instantiation is complete and that match occurs, the + least significant bit of the reachability register (peer.reach) is + set, indicating the peer is now reachable. The following peer + variables are copied (rescaled as necessary) from the NTP packet + variables and system variables: + + + + + + + + + + + +Mills [Page 25] + +RFC 1059 Network Time Protocol July 1988 + + + peer.leap <- pkt.leap peer.distance <- pkt.distance + peer.stratum <- pkt.stratum peer.drift <- pkt.drift + peer.ppoll <- pkt.poll peer.refid <- pkt.refid + peer.precision <- pkt.precision peer.reftime <- pkt.reftime + peer.org <- pkt.xmt peer.rec <- sys.clock + peer.threshold <- max( min( peer.ppoll, peer.hpoll, + NTP.MAXPOLL), NTP.MINPOLL) + + If either or both the pkt.org or pkt.rec packet variables are zero, + the sender did not have reliable values for them, so the receive + procedure is terminated at this point. If both of these variables + are nonzero, the roundtrip delay and clock offset relative to the + peer are calculated as follows. Number the times of sending and + receiving NTP messages as shown in Figure 3.1 and let i be an even + integer. Then t(i-3), t(i-2) and t(i-1) and t(i) are the contents of + the pkt.org, pkt.rec, pkt.xmt and peer.rec variables respectively. + + | | + t(1) |------------------->| t(2) + | | + t(4) |<-------------------| t(3) + | | + t(5) |------------------->| t(6) + | | + t(8) |<-------------------| t(7) + | | + ... + Figure 3.1. Calculating Delay and Offset + + The roundtrip delay d and clock offset c of the receiving peer + relative to the sending peer is: + + d = (t(i) - t(i-3)) - (t(i-1) - t(i-2)) + c = [(t(i-2) - t(i-3)) + (t(i-1) - t(i))]/2 . + + This method amounts to a continuously sampled, returnable-time + system, which is used in some digital telephone networks. Among the + advantages are that the order and timing of the messages is + unimportant and that reliable delivery is not required. Obviously, + the accuracies achievable depend upon the statistical properties of + the outbound and inbound net paths. Further analysis and + experimental results bearing on this issue can be found in + Appendix D. + + The c and d values are then input to the clock filter algorithm to + produce the delay estimate (peer.delay) and offset estimate + (peer.offset) for the peer involved. If d becomes nonpositive due to + low delays, long polling intervals and high drift rates, it should be + + + +Mills [Page 26] + +RFC 1059 Network Time Protocol July 1988 + + + considered invalid; however, even under these conditions it may + still be useful to update the local clock and reduce the drift rate + to the point that d becomes positive again. Specification of the + clock filter algorithm is not an integral part of the NTP + specification; however, one found to work well in the Internet + environment is described in Section 4. + + When a primary clock is connected to the host, it is convenient to + incorporate its information into the data base as if the clock were + represented by an ordinary peer. The clocks are usually polled once + or twice a minute and the returned timecheck used to produce a new + update for the logical clock. The update procedure is then called + with the following assumed peer variables: + + peer.offset <- timecheck - sys.clock + peer.delay <- as determined + peer.dispersion <- 0 + peer.leap <- selected by operator, ordinarily 00 + peer.stratum <- 0 + peer.distance <- 0 + peer.refid <- ASCII identifier + peer.reftime <- timecheck + + In this case the peer.delay and peer.refid can be constants + reflecting the type and accuracy of the clock. By convention, the + value for peer.delay is ten times the expected mean error of the + clock, for instance, 10 milliseconds for a WWVB clock and 1000 + milliseconds for a less accurate WWV clock, but with a floor of 100 + milliseconds. Other peer variables such as the peer timer and + reachability register can be used to control the polling interval and + to confirm the clock is operating correctly. In this way the clock + filter and selection algorithms operate in the usual way and can be + used to mitigate the clock itself, should it appear to be operating + correctly, yet deliver bogus time. + +3.4.3. Update Procedure + + The update procedure is called when a new delay/offset estimate is + available. First, the clock selection algorithm determines the best + peer on the basis of estimated accuracy and reliability, which may + result in a new clock source (sys.peer). If sys.peer points to the + peer data structure with the just-updated estimates, the state + variables of that peer are used to update the system state variables + + + + + + + + +Mills [Page 27] + +RFC 1059 Network Time Protocol July 1988 + + + as follows: + + sys.leap <- peer.leap + sys.stratum <- peer.stratum + 1 + sys.distance <- peer.distance + peer.delay + sys.refid <- peer.srcadr + sys.reftime <- peer.rec + + Finally, the logical clock procedure is called with peer.offset as + argument to update the logical clock (sys.clock) and recompute the + estimated drift rate (sys.drift). It may happen that the logical + clock may be reset, rather than slewed to its final value. In this + case the peer variables of all reachable peers are are updated as + follows: + + peer.hpoll <- NTP.MINPOLL + peer.disp <- NTP.MAXDISP + peer.filter <- 0 (cleared) + peer.org <- 0 + peer.rec <- 0 + + and the clock selection algorithm is called again, which results in a + null clock source (sys.peer = 0). A new selection will occur when + the filters fill up again and the dispersion settles down. + + Specification of the clock selection algorithm and logical clock + procedure is not an integral part of the NTP specification. A clock + selection algorithm found to work well in the Internet environment is + described in Section 4, while a logical clock procedure is described + in Section 5. The clock selection algorithm described in Section 4 + usually picks the server at the highest stratum and minimum delay + among all those available, unless that server appears to be a + falseticker. The result is that the algorithms all work to build a + minimum-weight spanning tree relative to the primary servers and thus + a hierarchical master-slave system similar to those used by some + digital telephone networks. + + + + + + + + + + + + + + + +Mills [Page 28] + +RFC 1059 Network Time Protocol July 1988 + + +3.4.4. Initialization Procedures + + Upon reboot the NTP host initializes all system variables as follows: + + sys.clock <- best available estimate + sys.leap <- 11 (unsynchronized) + sys.stratum <- 0 (undefined) + sys.precision <- as required + sys.distance <- 0 (undefined) + sys.drift <- as determined + sys.refid <- 0 (undefined) + sys.reftime <- 0 (undefined) + + The logical clock sys.clock is presumably undefined at reboot; + however, in some designs such as the Fuzzball an estimate is + available from the reboot environment. The sys.precision variable is + determined by the intrinsic architecture of the local hardware clock. + The sys.drift variable is determined as a side effect of subsequent + logical clock updates, from whatever source. + + Next, an implementation-specific instantiation procedure is called + repeatedly to establish the set of client peers or symmetric (active) + peers which will actively probe other servers during regular + operation. The mode and addresses of these peers is determined using + information read during the reboot procedure or as the result of + operator commands. + +4. Filtering Algorithms + + A very important factor affecting the accuracy and reliability of + time distribution is the complex of algorithms used to deglitch and + smooth the offset estimates and to cast out outlyers due to failure + of the primary reference sources or propagation media. The + algorithms suggested in this section were developed and refined over + several years of operation in the Internet under widely varying net + configurations and utilizations. While these algorithms are believed + the best available at the present time, they are not an integral part + of the NTP specification. + + There are two algorithms described in the following, the clock filter + algorithm, which is used to select the best offset samples from a + given clock, and the clock selection algorithm, which is used to + select the best clock among a hierarchical set of clocks. + +4.1. Clock Filter Algorithm + + The clock filter algorithm is executed upon arrival of each NTP + message that results in new delay/offset sample pairs. New sample + + + +Mills [Page 29] + +RFC 1059 Network Time Protocol July 1988 + + + pairs are shifted into the filter register (peer.filter) from the + left end, causing first zeros then old sample pairs to shift off the + right end. Then those sample pairs in peer.filter with nonzero delay + are inserted on a temporary list and sorted in order of increasing + delay. The delay estimate (peer.delay) and offset estimate + (peer.offset) are chosen as the delay/offset values corresponding to + the minimum-delay sample. In case of ties an arbitrary choice is + made. + + The dispersion estimate (peer.dispersion) is then computed as the + weighted sum of the offsets in the list. Assume the list has + PEER.SHIFT entries, the first m of which contain valid samples in + order of increasing delay. If X(i) (0 =< i < PEER.SHIFT) is the + offset of the ith sample, then, + + d(i) = |X(i) - X(0)| if i < m and |X(i) - X(0)| < 2^15 + d(i) = 2^15 - 1 otherwise + + peer.dispersion = Sum(d(i)*w^i) , + (0 =< i < PEER.SHIFT) + + where w < 1 is a weighting factor experimentally adjusted to match + typical offset distributions. The peer.dispersion variable is + intended for use as a quality indicator, with increasing values + associated with decreasing quality. The intent is that samples with + a peer.dispersion exceeding a configuration threshold will not be + used in subsequent processing. The prototype implementation uses a + weighting factor w = 0.5, also called PEER.FILTER, and a threshold + PEER.THRESHOLD of 500 ms, which insures that all stages of + peer.filter are filled and contain offsets within a few seconds of + each other. + +4.2. Clock Selection Algorithm + + The clock selection algorithm uses the values of peer.delay, + peer.offset and peer.dispersion calculated by the clock filter + algorithm and is called when these values change or when the + reachability status changes. It constructs a list of candidate + estimates according to a set of criteria designed to maximize + accuracy and reliability, then sorts the list in order of estimated + precision. Finally, it repeatedly casts out outlyers on the basis of + dispersion until only a single candidate is left. + + The selection process operates on each peer in turn and inspects the + various data captured from the last received NTP message header, as + well as the latest clock filter estimates. It selects only those + peers for which the following criteria are satisfied: + + + + +Mills [Page 30] + +RFC 1059 Network Time Protocol July 1988 + + + 1. The peer must be reachable and operating in client or symmetric + modes. + + 2. The peer logical clock must be synchronized, as indicated by the + Leap Indicator bits being other than 11. + + 3. If the peer is operating at stratum two or greater, it must not + be synchronized to this host, which means its reference clock + identifier (peer.refid) must not match the Internet address of + this host. This is analogous to the split-horizon rule used in + some variants of the Bellman-Ford routing algorithm. + + 4. The sum of the peer synchronizing distance (peer.distance) plus + peer.delay must be less than 2^13 (8192) milliseconds. Also, the + peer stratum (peer.stratum) must be less than eight and + peer.dispersion must be less than a configured threshold + PEER.THRESHOLD (currently 500 ms). These range checks were + established through experience with the prototype implementation, + but may be changed in future. + + For each peer which satisfies the above criteria, a sixteen-bit + keyword is constructed, with the low-order thirteen bits the sum of + peer.distance plus peer.delay and the high-order three bits the + peer.stratum reduced by one and truncated to three bits (thus mapping + zero to seven). The keyword together with a pointer to the peer data + structure are inserted according to increasing keyword values and + truncated at a maximum of eight entries. The resulting list + represents the order in which peers should be chosen according to the + estimated precision of measurement. If no keywords are found, the + clock source variable (sys.peer) is set to zero and the algorithm + terminates. + + The final procedure is designed to detect falsetickers or other + conditions which might result in gross errors. Let m be the number + of samples remaining in the list. For each i (0 =< i < m) compute + the dispersion d(i) of the list relative to i: + + d(i) = Sum(|X(j) - X(i)|*w^j) , + (0 =< j < m) + + where w < 1 is a weighting factor experimentally adjusted for the + desired characteristic (see below). Then cast out the entry with + maximum d(i) or, in case of ties, the maximum i, and repeat the + procedure. When only a single entry remains in the list, sys.peer is + set as its peer data structure pointer and the peer.hpoll variable in + that structure is set to NTP.MINPOLL as required by the logical clock + mechanism described in Section 5. + + + + +Mills [Page 31] + +RFC 1059 Network Time Protocol July 1988 + + + This procedure is designed to favor those peers near the head of the + list, which are at the highest stratum and lowest delay and + presumably can provide the most precise time. With proper selection + of weighting factor w, also called PEER.SELECT, entries will be + trimmed from the tail of the list, unless a few outlyers disagree + significantly with respect to the remaining entries, in which case + the outlyers are discarded first. + + In order to see how this procedure works to select outlyers, consider + the case of three entries and assume that one or more of the offsets + are clustered about zero and others are clustered about one. For w = + 0.75 as used in the prototype implementations and multiplying by 16 + for convenience, the first entry has weight w^0 = 16, the second w^1 + = 12 and the third w^2 = 9. Table X shows for all combinations of + peer offsets the calculated dispersion about each of the three + entries, along with the results of the procedure. + + Peer 0 1 2 Dispersion Cast Result + Weight 16 12 9 0 1 2 Out + ------------------------------------------------------ + 0 0 0 0 0 0 2 0 0 + 0 0 1 9 9 28 2 0 0 + 0 1 0 12 25 12 1 0 0 + 0 1 1 21 16 16 0 1 1 + 1 0 0 21 16 16 0 0 0 + 1 0 1 12 25 12 1 1 1 + 1 1 0 9 9 28 2 1 1 + 1 1 1 0 0 0 2 1 1 + + Table 4.1. Outlyer Selection Procedure + + In the four cases where peer 0 and peer 1 disagree, the outcome is + determined by peer 2. Similar outcomes occur in the case of four + peers. While these outcomes depend on judicious choice of w, the + behavior of the algorithm is substantially the same for values of w + between 0.5 and 1.0. + +4.3. Variable-Rate Polling + + As NTP service matures in the Internet, the resulting network traffic + can become burdensome, especially in the primary service net. In + this expectation, it is useful to explore variable-rate polling, in + which the intervals between NTP messages can be adjusted to fit + prevailing network conditions of delay dispersion and loss rate. The + prototype NTP implementation uses this technique to reduce the + network overheads to one-sixteenth the maximum rate, depending on + observed dispersion and loss. + + + + +Mills [Page 32] + +RFC 1059 Network Time Protocol July 1988 + + + The prototype implementation adjusts the polling interval peer.hpoll + in response to the reachability register (peer.reach) variable along + with the dispersion (peer.dispersion) variable. So long as the clock + source variable (sys.peer) does not point to the peer data structure, + peer.reach is nonzero (reachable) and peer.dispersion is less than + the PEER.THRESHOLD parameter, the value of peer.hpoll is increased by + one for each call on the update procedure, subject to a maximum of + NTP.MAXPOLL. Following the timeout procedure, if peer.reach + indicates messages have not been received for the preceding two + polling intervals (low-order two bits are zero), the value of + peer.hpoll is decreased by one, subject to a minimum of NTP.MINPOLL. + If peer.reach becomes zero (unreachable), the value of peer.hpoll is + set to NTP.MINPOLL. + + The result of the above mechanism is that the polling intervals for + peers not selected for synchronization and in symmetric mode creep + upwards once the filter register (peer.filter) has filled and the + peer.dispersion has settled down, but decrease again in case + peer.dispersion increases or the loss rate increases or the peer + becomes unreachable. + +5. Logical Clocks + + In order to implement a logical clock, the host must be equipped with + a hardware clock consisting of an oscillator and interface and + capable of the required precision and stability. The logical clock + is adjusted by means of periodic offset corrections computed by NTP + or some other time-synchronization protocol such as Hellospeak [15] + or the Unix 4.3bsd TSP [20]. Following is a description of the + Fuzzball logical clock, which includes provisions for precise time + and frequency adjustment and can maintain time to within a + millisecond and frequency to within a day per millisecond. + + The logical clock is implemented using a 48-bit Clock Register, which + increments at 1000-Hz (at the decimal point), a 32-bit Clock-Adjust + Register, which is used to slew the Clock Register in response to + offset corrections, and a Drift-Compensation Register, which is used + to trim the oscillator frequency. In some interface designs such as + the DEC KWV11, an additional hardware register, the Counter Register, + is used as an auxiliary counter. The configuration and decimal point + of these registers are shown in Figure 5.1. + + + + + + + + + + +Mills [Page 33] + +RFC 1059 Network Time Protocol July 1988 + + + Clock Register + + 0 16 32 + +---------------+---------------+---------------+ + | | | | + +---------------+---------------+---------------+ + A + decimal point + + Clock-Adjust Register + + 0 16 + +---------------+---------------+ + | | | + +---------------+---------------+ + A + decimal point + + Drift-Compensation Register + + 0 16 + +---------------+ + | | + +---------------+ + A + decimal point + + Counter Register + + 0 16 + +---------------+ + | | + +---------------+ + A + decimal point + + Figure 5.1. Clock Registers + + The Clock Register, Clock-Adjust Register and Drift-Compensation + Register are implemented in memory. In typical clock interface + designs such as the DEC KWV11, the Counter Register is implemented as + a buffered counter driven by a crystal oscillator. A counter + overflow is signalled by an interrupt, which results in an increment + of the Clock Register at bit 15 and the propagation of carries as + required. The time of day is determined by reading the Counter + Register, which does not disturb the counting process, and adding its + value to that of the Clock Register with decimal points aligned. + + + + +Mills [Page 34] + +RFC 1059 Network Time Protocol July 1988 + + + In other interface designs such as the LSI-11 event-line mechanism, + each tick of the clock is signalled by an interrupt at intervals of + 16-2/3 or 20 ms, depending on interface and mains frequency. When + this occurs the appropriate increment in milliseconds, expressed to + 32 bits in precision, is added to the Clock Register with decimal + points aligned. + +5.1. Uniform Phase Adjustments + + Left uncorrected, the logical clock runs at the rate of its intrinsic + oscillator. A correction is introduced as a signed 32-bit integer in + milliseconds, which is added to the Drift-Compensation Register and + also replaces bits 0-15 of the Clock-Adjust Register, with bits 16-31 + set to zero. At adjustment intervals of CLOCK.ADJ a correction + consisting of two components is computed. The first (phase) + component consists of the Clock-Adjust Register shifted right + CLOCK.PHASE bits, which is then subtracted from the Clock-Adjust + Register. The second (frequency) component consists of the Drift- + Compensation Register shifted right CLOCK.FREQ bits. The sum of the + phase and frequency components is the correction, which is then added + to the Clock Register. Operation continues in this way until a new + correction is introduced. + + Care is required in the implementation to insure monotonicity of the + Clock Register and to preserve the highest precision while minimizing + the propagation of roundoff errors. This can be done by buffering + the corrections and adding them to the increment at the time the + Clock Register is next updated. Monotonicity is insured with the + parameters shown in Table 5.1, as long as the increment is at least 2 + ms. This table shows the above parameters and others discussed below + for both a crystal-stabilized oscillator and a mains-frequency + oscillator. + + Parameter Name Crystal Mains + ------------------------------------------------------------------- + Update Interval CLOCK.ADJ 4 sec 1 sec + Phase Shift CLOCK.PHASE -8 -9 + Frequency Shift CLOCK.FREQ -16 -16 + Maximum Aperture CLOCK.MAX +-128 ms +-256 ms + Shift Register Size PEER.SHIFT 8 4 + Host Poll Interval peer.hpoll NTP.MINPOLL NTP.MINPOLL + (64 sec) (64 sec) + + Table 5.1. Clock Parameters + + The above design constitutes a second-order phase-lock loop which + adjusts the logical clock phase and frequency to compensate for the + intrinsic oscillator jitter, wander and drift. Simulation of a loop + + + +Mills [Page 35] + +RFC 1059 Network Time Protocol July 1988 + + + with parameters chosen from Table 5.1 for a crystal-stabilized + oscillator and the clock filter described in Section 4 results in the + following transient response: For a phase correction of 100 ms the + loop reaches zero error in 34 minutes, overshoots 7 ms in 76 minutes + and settles to less than 1 ms in about four hours. The maximum + frequency error is about 6 ppm at 40 minutes and returns to less than + 1 ppm in about eight hours. For a frequency correction of 10 ppm the + loop settles to within 1 ppm in about nine hours and to within 0.1 + ppm in about a day. These characteristics are appropriate for + typical computing equipment using board-mounted crystals without oven + temperature control. + + In those cases where mains-frequency oscillators must be used, the + loop parameters must be adapted for the relatively high jitter and + wander characteristics of the national power grid, in which diurnal + peak-to-peak phase excursions can exceed four seconds. Simulation of + a loop with parameters chosen from Table 5.1 for a mains-frequency + oscillator and the clock filter described in Section 4 results in a + transient response similar to the crystal-stabilized case, but with + time constants only one-fourth those in that case. When presented + with actual phase-offset data for typical Summer days when the jitter + and wander are the largest, the loop errors are in the order of a few + tens of milliseconds, but not greater than 150 ms. + + The above simulations assume the clock filter algorithm operates to + select the oldest sample in the shift register at each step; that + is, the filter operates as a delay line with delay equal to the + polling interval times the number of stages. This is a worst-case + scenario, since the larger the overall delay the harder it is to + maintain low loop errors together with good transient response. The + parameters in Table 5.1 were experimentally determined with this + scenario and the constraint that the polling interval could not be + reduced below 64 seconds. With these parameters it is not possible + to increase the polling interval above 64 seconds without significant + increase in loop error or degradation of transient response. Thus, + when a clock is selected according to the algorithms of Section 4, + the polling interval peer.hpoll is always set at NTP.MINPOLL. + +5.2. Nonuniform Phase Adjustments + + When the magnitude of a correction exceeds a maximum aperture + CLOCK.MAX, the possibility exists that the clock is so far out of + synchronization with the reference source that the best action is an + immediate and wholesale replacement of Clock Register contents, + rather than a graduated slewing as described above. In practice the + necessity to do this is rare and occurs when the local host or + reference source is rebooted, for example. This is fortunate, since + step changes in the clock can result in the clock apparently running + + + +Mills [Page 36] + +RFC 1059 Network Time Protocol July 1988 + + + backward, as well as incorrect delay and offset measurements of the + synchronization mechanism itself. + + Considerable experience with the Internet environment suggests the + values of CLOCK.MAX tabulated in Table 5.1 as appropriate. In + practice, these values are exceeded with a single time-server source + only under conditions of the most extreme congestion or when multiple + failures of nodes or links have occured. The most common case when + the maximum is exceeded is when the time-server source is changed and + the time indicated by the new and old sources exceeds the maximum due + to systematic errors in the primary reference source or large + differences in the synchronizing path delays. + +5.3. Maintaining Date and Time + + Conversion from NTP format to the common date and time formats used + by application programs is simplified if the internal local-clock + format uses separate date and time registers. The time register is + designed to roll over at 24 hours, give or take a leap second as + determined by the Leap Indicator bits, with its overflows + (underflows) incrementing (decrementing) the date register. The date + and time registers then indicate the number of days and seconds since + some previous reference time, but uncorrected for leap seconds. + + On the day prior to the insertion of a leap second the Leap Indicator + bits are set at the primary servers, presumably by manual means. + Subsequently, these bits show up at the local host and are passed to + the logical clock procedure. This causes the modulus of the time + register, which is the length of the current day, to be increased or + decreased by one second as appropriate. On the day following + insertion the bits are turned off at the primary servers. While it + is possible to turn the bits off automatically, the procedure + suggested here insures that all clocks have rolled over and will not + be reset incorrectly to the previous day as the result of possible + corrections near the instant of rollover. + +5.4. Estimating Errors + + After an NTP message is received and until the next one is received, + the accuracy of the local clock can be expected to degrade somewhat. + The magnitude of this degradation depends on the error at the last + update time together with the drift of the local oscillator with + respect to time. It is possible to estimate both the error and drift + rate from data collected during regular operation. These data can be + used to determine the rate at which NTP neighbors should exchange NTP + messages and thus control net overheads. + + NTP messages include the local-clock precision of the sender, as well + + + +Mills [Page 37] + +RFC 1059 Network Time Protocol July 1988 + + + as the reference time, estimated drift and a quantity called the + synchronizing distance. The precision of the local clock, together + with its peer clocks, establishes the short-term jitter + characteristics of the offset estimates. The reference time and + estimated drift of the sender provide an error estimate at the time + the latest update was received. The synchronizing distance provides + an estimate of error relative to the primary reference source and is + used by the filtering algorithms to improve the quality and + reliability of the offset estimates. + + Estimates of error and drift rate are not essential for the correct + functioning of the clock algorithms, but do improve the accuracy and + adjustment with respect to net overheads. The estimated error allows + the recipient to compute the rate at which independent samples are + required in order to maintain a specified estimated error. The + estimated drift rate allows the recipient to estimate the optimum + polling interval. + + It is possible to compute the estimated drift rate of the local clock + to a high degree of precision by simply adding the n offsets received + during an interval T to an accumulator. If X1 and X2 are the values + of the accumulator at the beginning and end of T, then the estimated + drift rate r is: + + X2 - X1 n + r = ------- --- . + n T + + The intrinsic (uncorrected) drift rate of typical crystal oscillators + under room-temperature conditions is in the order of from a few parts + per million (ppm) to as much as 100 ppm, or up to a few seconds per + day. For most purposes the drift of a particular crystal oscillator + is constant to within perhaps one ppm. Assuming T can be estimated + to within 100 ms, for example, it would take about a day of + accumulation to estimate r to an uncertainty in the order of one ppm. + + Some idea of the estimated error of the local clock can be derived + from the variance of the offsets about the mean per unit time. This + can be computed by adding the n offset squares received during T to + an accumulator. If Y1 and Y2 are the values of the accumulator at + the beginning and end of T, then the estimated error s is: + + Y2 - Y1 (X2 - X1)^2 n + s = ( ------- - ----------- ) --- . + n n * n T + + The quantities r and s have direct utility to the peer as noted + above. However, they also have indirect utility to the recipient of + + + +Mills [Page 38] + +RFC 1059 Network Time Protocol July 1988 + + + an NTP message sent by that peer, since they can be used as weights + in such algorithms as described in [22], as well as to improve the + estimates during periods when offsets are not available. It is most + useful if the latest estimate of these quantities are available in + each NTP message sent; however, considerable latitude remains in the + details of computation and storage. + + The above formulae for r and s imply equal weighting for offsets + received throughout the accumulation interval T. One way to do this + is using a software shift register implemented as a circular buffer. + A single pointer points to the active entry in the buffer and + advances around one entry as each new offset is stored. There are + two accumulators, one for the offset and the other for its squares. + When a new offset arrives, a quantity equal to the new offset minus + the old (active) entry is added to the first accumulator and the + square of this quantity is added to the second. Finally, the offset + is stored in the circular buffer. + + The size of the circular buffer depends on the accumulation interval + T and the rate offsets are produced. In many reachability and + routing algorithms, such as GGP, EGP and local-net control + algorithms, peers exchange messages on the order of once or twice a + minute. If NTP peers exchanged messages at a rate of one per minute + and if T were one day, the circular buffer would have to be 1440 + words long; however, a less costly design might aggregate the data + in something like half-hour segments, which would reduce the length + of the buffer to 48 words while not significantly affecting the + quality of the data. + + + + + + + + + + + + + + + + + + + + + + + +Mills [Page 39] + +RFC 1059 Network Time Protocol July 1988 + + +6. References + + 1. Lamport, L., "Time, Clocks and the Ordering of Events in a + Distributed System", Communications of the ACM, Vol. 21, No. 7, + pgs. 558-565, July 1978. + + 2. "Time and Frequency Dissemination Services", NBS Special + Publication No. 432, US Department of Commerce, 1979. + + 3. Lindsay, W., and A. Kantak, "Network Synchronization of Random + Signals", IEEE Trans. Comm., COM-28, No. 8, pgs. 1260-1266, + August 1980. + + 4. Braun, W., "Short Term Frequency Effects in Networks of Coupled + Oscillators", IEEE Trans. Comm., COM-28, No. 8, pgs. 1269-1275, + August 1980. + + 5. Mitra, D., "Network Synchronization: Analysis of a Hybrid of + Master-Slave and Mutual Synchronization", IEEE Trans. Comm. + COM-28, No. 8, pgs. 1245-1259, August 1980. + + 6. Postel, J., "User Datagram Protocol", RFC-768, USC/Information + Sciences Institute, August 1980. + + 7. Mills, D., "Time Synchronization in DCNET Hosts", IEN-173, COMSAT + Laboratories, February 1981. + + 8. Mills, D., "DCNET Internet Clock Service", RFC-778, COMSAT + Laboratories, April 1981. + + 9. Su, Z., "A Specification of the Internet Protocol (IP) Timestamp + Option", RFC-781, SRI International, May 1981. + + 10. Defense Advanced Research Projects Agency, "Internet Protocol", + RFC-791, USC/Information Sciences Institute, September 1981. + + 11. Defense Advanced Research Projects Agency, "Internet Control + Message Protocol", RFC-792, USC/Information Sciences Institute, + September 1981. + + 12. Postel, J., "Daytime Protocol", RFC-867, USC/Information Sciences + Institute, May 1983. + + 13. Postel, J., "Time Protocol", RFC-868, USC/Information Sciences + Institute, May 1983. + + 14. Mills, D., "Internet Delay Experiments", RFC-889, M/A-COM + Linkabit, December 1983. + + + +Mills [Page 40] + +RFC 1059 Network Time Protocol July 1988 + + + 15. Mills, D., "DCN Local-Network Protocols", RFC-891, M/A-COM + Linkabit, December 1983. + + 16. Gusella, R., and S. Zatti, "TEMPO - A Network Time Controller for + a Distributed Berkeley UNIX System", IEEE Distributed Processing + Technical Committee Newsletter 6, No. SI-2, pgs. 7-15, June 1984. + Also in: Proc. Summer 1984 USENIX, Salt Lake City, June 1984. + + 17. Halpern, J., Simons, B., Strong, R., and D. Dolly, "Fault- + Tolerant Clock Synchronization", Proc. Third Annual ACM Symposium + on Principles of Distributed Computing, pgs. 89-102, August 1984. + + 18. Lundelius, J., and N. Lynch, "A New Fault-Tolerant Algorithm for + Clock Synchronization:, Proc. Third Annual ACM Symposium on + Principles of Distributed Computing, pgs. 75-88, August 1984. + + 19. Lamport, L., and P. Melliar-Smith "Synchronizing Clocks in the + Presence of Faults", JACM 32, No. 1, pgs. 52-78, January 1985. + + 20. Gusella, R., and S. Zatti, "The Berkeley UNIX 4.3BSD Time + Synchronization Protocol: Protocol Specification", Technical + Report UCB/CSD 85/250, University of California, Berkeley, June + 1985. + + 21. Marzullo, K., and S. Owicki, "Maintaining the Time in a + Distributed System", ACM Operating Systems Review 19, No. 3, pgs. + 44-54, July 1985. + + 22. Mills, D., "Algorithms for Synchronizing Network Clocks", RFC- + 956, M/A-COM Linkabit, September 1985. + + 23. Mills, D., "Experiments in Network Clock Synchronization", RFC- + 957, M/A-COM Linkabit, September 1985. + + 24. Mills, D., "Network Time Protocol (NTP)", RFC-958, M/A-COM + Linkabit, September 1985. + + 25. Gusella, R., and S. Zatti, "An Election Algorithm for a + Distributed Clock Synchronization Program", Technical Report + UCB/CSD 86/275, University of California, Berkeley, December + 1985. + + 26. Sams, H., "Reference Data for Engineers: Radio, Electronics, + Computer and Communications (Seventh Edition)", Indianapolis, + 1985. + + 27. Schneider, F., "A Paradigm for Reliable Clock Synchronization", + Technical Report TR 86-735, Cornell University, February 1986. + + + +Mills [Page 41] + +RFC 1059 Network Time Protocol July 1988 + + + 28. Tripathi, S., and S. Chang, "ETempo: A Clock Synchronization + Algorithm for Hierarchical LANs - Implementation and + Measurements", Systems Research Center Technical Report TR-86-48, + University of Maryland, 1986. + + 29. Bertsekas, D., and R. Gallager, "Data Networks", Prentice-Hall, + Englewood Cliffs, NJ, 1987. + + 30. Srikanth, T., and S. Toueg. "Optimal Clock Synchronization", JACM + 34, No. 3, pgs. 626-645, July 1987. + + 31. Rickert, N., "Non Byzantine Clock Synchronization - A Programming + Experiment", ACM Operating Systems Review 22, No. 1, pgs. 73-78, + January 1988. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Mills [Page 42] + +RFC 1059 Network Time Protocol July 1988 + + +Appendix A. UDP Header Format + + An NTP packet consists of the UDP header followed by the NTP data + portion. The format of the UDP header and the interpretation of its + fields are described in [6] and are not part of the NTP + specification. They are shown below for completeness. + + 0 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Source Port | Destination Port | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Length | Checksum | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + Source Port + + UDP source port number. In the case of a client request this + field is assigned by the client host, while for a server reply + it is copied from the Destination Port field of the client + request. In the case of symmetric mode, both the Source Port + and Destination Port fields are assigned the NTP service-port + number 123. + + Destination Port + + UDP destination port number. In the case of a client request + this field is assigned the NTP service-port number 123, while + for a server reply it is copied from the Source Port field of + the client request. In the case of symmetric mode, both the + Source Port and Destination Port fields are assigned the NTP + service-port number 123. + + Length + + Length of the request or reply, including UDP header, in + octets + + Checksum + + Standard UDP checksum + + + + + + + + + + +Mills [Page 43] + +RFC 1059 Network Time Protocol July 1988 + + +Appendix B. NTP Data Format - Version 1 + + The format of the NTP data portion, which immediately follows the UDP + header, is shown below along with a description of its fields. + + + 0 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |LI | VN |0 0 0| Stratum | Poll | Precision | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Synchronizing Distance | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Estimated Drift Rate | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Reference Clock Identifier | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | + | Reference Timestamp (64 bits) | + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | + | Originate Timestamp (64 bits) | + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | + | Receive Timestamp (64 bits) | + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | + | Transmit Timestamp (64 bits) | + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + Leap Indicator (LI) + + Two-bit code warning of impending leap-second to be inserted + at the end of the last day of the current month. Bits are + coded as follows: + + 00 no warning + 01 +1 second (following minute has 61 seconds) + 10 -1 second (following minute has 59 seconds) + 11 alarm condition (clock not synchronized) + + + + + + + +Mills [Page 44] + +RFC 1059 Network Time Protocol July 1988 + + + Version Number (VN) + + Three-bit code indicating the version number, currently one + (1). + + Reserved + + Three-bit field consisting of all zeros and reserved for + future use. + + Stratum + + Integer identifying stratum level of local clock. Values are + defined as follows: + + 0 unspecified + 1 primary reference (e.g., radio clock) + 2...n secondary reference (via NTP) + + Poll + + Signed integer indicating the maximum interval between + successive messages, in seconds to the nearest power of two. + + Precision + + Signed integer indicating the precision of the local clock, in + seconds to the nearest power of two. + + Synchronizing Distance + + Fixed-point number indicating the estimated roundtrip delay to + the primary synchronizing source, in seconds with fraction + point between bits 15 and 16. + + Estimated Drift Rate + + Fixed-point number indicating the estimated drift rate of the + local clock, in dimensionless units with fraction point to the + left of the most significant bit. + + Reference Clock Identifier + + Code identifying the particular reference clock. In the case + of type 0 (unspecified) or type 1 (primary reference), this is + a left-justified, zero-filled ASCII string, for example: + + + + + +Mills [Page 45] + +RFC 1059 Network Time Protocol July 1988 + + + Type Code Meaning + --------------------------------------------------- + 0 DCN Determined by DCN routing algorithm + 1 WWVB WWVB radio clock (60 kHz) + 1 GOES GOES satellite clock (468 MHz) + 1 WWV WWV radio clock (5/10/15 MHz) + (and others as necessary) + + In the case of type 2 and greater (secondary reference), this + is the 32-bit Internet address of the reference host. + + Reference Timestamp + + Local time at which the local clock was last set or corrected. + + Originate Timestamp + + Local time at which the request departed the client host for + the service host. + + Receive Timestamp + + Local time at which the request arrived at the service host. + + Transmit Timestamp + + Local time at which the reply departed the service host for + the client host. + + + + + + + + + + + + + + + + + + + + + + + +Mills [Page 46] + +RFC 1059 Network Time Protocol July 1988 + + +Appendix C. Timeteller Experiments + + In order to update data collected in June 1985 and reported in RFC- + 957, a glorious three-day experiment was carried out in January 1988 + with all the hosts and gateways listed in the NIC data base. Four + packets were sent at five-second intervals to each host and gateway + using UDP/NTP, UDP/TIME and ICMP/TIMESTAMP protocols and the clock + offsets (in milliseconds) for each protocol averaged with respect to + local time, which is synchronized via NTP to a radio-clock host. + While the ICMP/TIMESTAMP protocol has much finer granularity + (milliseconds) than UDP/TIME (seconds), it has no provisions for the + date, so is not suitable as a time-synchronization protocol; + however, it was included in the experiments both as a sanity check + and in order to assess the precision of measurement. + + In the latest survey of 5498 hosts and 224 gateways, 46 responded to + UDP/NTP requests, 1158 to UDP/TIME and 1963 to ICMP/TIMESTAMP. By + contrast, in the 1985 survey of 1775 hosts and 110 gateways, 163 + responded to UDP/TIME requests and 504 to ICMP/TIMESTAMP. At that + time there were no UDP/NTP implementations. There are many more + hosts and gateways listed in the rapidly growing domain-name system, + but not listed in the NIC data base, and therefore not surveyed. The + results of the survey are given in Table C.1, which shows for each of + the three protocols the error X for which the distribution function + P[x =< X] has the value shown. + + P[x=<X] UDP/NTP UDP/TIME ICMP/TIMESTAMP + ------------------------------------------------------ + .1 11 4632 5698 + .2 37 18238 27965 + .3 66 38842 68596 + .4 177 68213 127367 + .5 364 126232 201908 + .6 567 195950 285092 + .7 3466 267119 525509 + .8 20149 422129 2.91426E+06 + .9 434634 807135 5.02336E+07 + 1 1.17971E+09 1.59524E+09 2.11591E+09 + + Table C.1. Distribution Functions + + It can be seen that ten percent of the UDP/NTP responses show errors + of 11 milliseconds or less and that ten percent of the UDP/TIME + responses show errors greater than 807135 milliseconds (about 13 + minutes). Fifty percent of the UDP/NTP timetellers are within 364 + milliseconds, while fifty percent of the UDP/TIME tellers are within + 126232 milliseconds (just over two minutes). Surprisingly, + ICMP/TIMESTAMP responses show errors even larger than UDP/TIME. + + + +Mills [Page 47] + +RFC 1059 Network Time Protocol July 1988 + + + However, the maximum error shown in all three protocols exceeded the + range that could be recorded, in this case about 12 days. Clearly, + there are good timetellers and bad. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Mills [Page 48] + +RFC 1059 Network Time Protocol July 1988 + + +Appendix D. Evaluation of Filtering Algorithms + + A number of algorithms for deglitching and filtering time-offset data + were described in RFC-956. These fall in two classes: majority- + subset algorithms, which attempt to separate good subsets from bad by + comparing their means, and clustering algorithms, which attempt to + improve the estimate by repeatedly casting out outlyers. The former + class was suggested as a technique to select the best (i.e. the most + reliable) clocks from a population, while the latter class was + suggested as a technique to improve the offset estimate for a single + clock given a series of observations. + + Following publication of RFC-956 and after further development and + experimentation using typical Internet paths, a better algorithm was + found for casting out outlyers from a continuous stream of offset + observations spaced at intervals in the order of minutes. The + algorithm is described as a variant of a median filter, in which a + window consisting of the last n sample offsets is continuously + updated and the median sample selected as the estimate. However, in + the modified algorithm the outlyer (sample furthest from the median) + is then discarded and the entire process repeated until only a single + sample offset is left, which is then selected as the estimate. + + The modified algorithm was found to be more resistant to glitches and + to provide a more accurate estimate than the unmodified one. It has + been implemented in the NTP daemons developed for the Fuzzball and + Unix operating systems and been in regular operation for about two + years. However, recent experiments have shown there is an even + better one which provides comparable accuracy together with a much + lower computational burden. The key to the new algorithm became + evident through an examination of scatter diagrams plotting sample + offset versus roundtrip delay. + + To see how a scatter diagram is constructed, it will be useful to + consider how offsets and delays are computed. Number the times of + sending and receiving NTP messages as shown in Figure D.1 and let i + be an even integer. Then the timestamps t(i-3), t(i-2) and t(i-1) + and t(i) are sufficient to calculate the offset and delay of each + peer relative to the other. + + + + + + + + + + + + +Mills [Page 49] + +RFC 1059 Network Time Protocol July 1988 + + + Peer 1 Peer 2 + | | + t(1) |------------------->| t(2) + | | + t(4) |<-------------------| t(3) + | | + t(5) |------------------->| t(6) + | | + t(8) |<-------------------| t(7) + | | + ... + + Figure D.1. Calculating Delay and Offset + + The roundtrip delay d and clock offset c of the receiving peer + relative to the sending peer are: + + + d = (t(i) - t(i-3)) - (t(i-1) - t(i-2)) + c = [(t(i-2) - t(i-3)) + (t(i-1) - t(i))]/2 . + + Two implicit assumptions in the above are that the delay distribution + is independent of direction and that the intrinsic drift rates of the + client and server clocks are small and close to the same value. If + this is the case the scatter diagram would show the samples + concentrated about a horizontal line extending from the point (d,c) + to the right. However, this is not generally the case. The typical + diagram shows the samples dispersed in a wedge with apex (d,c) and + opening to the right. The limits of the wedge are determined by + lines extending from (d,c) with slopes +0.5 and -0.5, which + correspond to the locus of points as the delay in one direction + increases while the delay in the other direction does not. In some + cases the points are concentrated along these two extrema lines, with + relatively few points remaining within the opening of the wedge, + which would correspond to increased delays on both directions. + + Upon reflection, the reason for the particular dispersion shown in + the scatter diagram is obvious. Packet-switching nets are most often + operated with relatively small mean queue lengths in the order of + one, which means the queues are often idle for relatively long + periods. In addition, the routing algorithm most often operates to + minimize the number of packet-switch hops and thus the number of + queues. Thus, not only is the probability that an arriving NTP + packet finds a busy queue in one direction reasonably low, but the + probability of it finding a busy queue in both directions is even + lower. + + From the above discussion one would expect that, at low utilizations + + + +Mills [Page 50] + +RFC 1059 Network Time Protocol July 1988 + + + and hop counts the points should be concentrated about the apex of + the wedge and begin to extend rightward along the extrema lines as + the utilizations and hop counts increase. As the utilizations and + hop counts continue to increase, the points should begin to fill in + the wedge as it expands even further rightward. This behavior is in + fact what is observed on typical Internet paths involving ARPANET, + NSFNET and other nets. + + These observations cast doubt on the median-filter approach as a good + way to cast out offset outlyers and suggests another approach which + might be called a minimum filter. From the scatter diagrams it is + obvious that the best offset samples occur at the lower delays. + Therefore, an appropriate technique would be simply to select from + the n most recent samples the sample with lowest delay and use its + associated offset as the estimate. An experiment was designed to + test this technique using measurements between selected hosts + equipped with radio clocks, so that delays and offsets could be + determined independent of the measurement procedure itself. + + The raw delays and offsets were measured by NTP from hosts at U + Maryland (UMD) and U Delaware (UDEL) via net paths to each other and + other hosts at Ford Research (FORD), Information Sciences Institute + (ISI) and National Center for Atmospheric Research (NCAR). For the + purposes here, all hosts can be assumed synchronized to within a few + milliseconds to NBS time, so that the delays and offsets reflect only + the net paths themselves. + + The results of the measurements are given in Table D.1 (UMD) and + Table D.2 (UDEL), which show for each of the paths the error X for + which the distribution function P[x =< X] has the value shown. Note + that the values of the distribution function are shown by intervals + of decreasing size as the function increases, so that its behavior in + the interesting regime of low error probability can be more + accurately determined. + + + + + + + + + + + + + + + + + +Mills [Page 51] + +RFC 1059 Network Time Protocol July 1988 + + + UMD FORD ISI NCAR UMD FORD ISI NCAR + Delay 1525 2174 1423 Offset 1525 2174 1423 + --------------------------- --------------------------- + .1 493 688 176 .1 2 17 1 + .2 494 748 179 .2 4 33 2 + .3 495 815 187 .3 9 62 3 + .4 495 931 205 .4 18 96 8 + .5 497 1013 224 .5 183 127 13 + .6 503 1098 243 .6 4.88E+8 151 20 + .7 551 1259 265 .7 4.88E+8 195 26 + .8 725 1658 293 .8 4.88E+8 347 35 + .9 968 2523 335 .9 4.88E+8 775 53 + .99 1409 6983 472 .99 4.88E+8 2785 114 + .999 14800 11464 22731 .999 4.88E+8 5188 11279 + 1 18395 15892 25647 1 4.88E+8 6111 12733 + + Table D.1. Delay and Offset Measurements (UMD) + + UDEL FORD UMD ISI NCAR + Delay 2986 3442 3215 2756 + ----------------------------------- + .1 650 222 411 476 + .2 666 231 436 512 + .3 692 242 471 554 + .4 736 256 529 594 + .5 787 272 618 648 + .6 873 298 681 710 + .7 1013 355 735 815 + .8 1216 532 845 1011 + .9 1836 1455 1019 1992 + .99 4690 3920 1562 4334 + .999 15371 6132 2387 11234 + 1 21984 8942 4483 21427 + + Table D.2.a Delay Measurements (UDEL) + + + + + + + + + + + + + + + + +Mills [Page 52] + +RFC 1059 Network Time Protocol July 1988 + + + UDEL FORD UMD ISI NCAR + Offset 2986 3442 3215 2756 + ----------------------------------- + .1 83 2 16 12 + .2 96 5 27 24 + .3 108 9 36 36 + .4 133 13 48 51 + .5 173 20 67 69 + .6 254 30 93 93 + .7 429 51 130 133 + .8 1824 133 165 215 + .9 4.88E+8 582 221 589 + .99 4.88E+8 1757 539 1640 + .999 4.88E+8 2945 929 5278 + 1 5.63E+8 4374 1263 10425 + + Table D.2.b Offset Measurements (UDEL) + + The results suggest that accuracies less than a few seconds can + usually be achieved for all but one percent of the measurements, but + that accuracies degrade drastically when the remaining measurements + are included. Note that in the case of the UMD measurements to FORD + almost half the measurements showed gross errors, which was due to + equipment failure at that site. These data were intentionally left + in the sample set to see how well the algorithms dealt with the + problem. + + The next two tables compare the results of minimum filters (Table + D.3) and median filters (Table D.4) for various n when presented with + the UMD - - NCAR raw sample data. The results show consistently + lower errors for the minimum filter when compared with the median + filter of nearest value of n. Perhaps the most dramatic result of + both filters is the greatly reduced error at the upper end of the + range. In fact, using either filter with n at least three results in + no errors greater than 100 milliseconds. + + + + + + + + + + + + + + + + +Mills [Page 53] + +RFC 1059 Network Time Protocol July 1988 + + + Filter Samples + 1 2 4 8 16 + P[x=<X] 1423 1422 1422 1420 1416 + - -------------------------------------------- + .1 1 1 1 0 0 + .2 2 1 1 1 1 + .3 3 2 1 1 1 + .4 8 2 2 1 1 + .5 13 5 2 2 1 + .6 20 10 3 2 2 + .7 26 15 6 2 2 + .8 35 23 11 4 2 + .9 53 33 20 9 3 + .99 114 62 43 28 23 + .999 11279 82 57 37 23 + 1 12733 108 59 37 23 + + Table D.3. Minimum Filter + (UMD - NCAR) + + Filter Samples + 3 7 15 + P[x=<X] 1423 1423 1423 + ---------------------------- + .1 2 2 2 + .2 2 4 5 + .3 5 8 8 + .4 10 11 11 + .5 13 14 14 + .6 18 17 16 + .7 23 21 19 + .8 28 25 23 + .9 36 30 27 + .99 64 46 35 + .999 82 53 44 + 1 82 60 44 + + Table D.4. Median Filter + (UMD - NCAR) + + While the UMD - NCAR data above represented a path across the NSFNET + Backbone, which normally involves only a few hops via Ethernets and + 56-Kbps links, the UDEL - NCAR path involves additional ARPANET hops, + which can contribute substantial additional delay dispersion. The + following Table D.5. shows the results of a minimum filter for + various n when presented with the UDEL - NCAR raw sample data. The + range of error is markedly greater than the UMD - NCAR path above, + especially near the upper end of the distribution function. + + + +Mills [Page 54] + +RFC 1059 Network Time Protocol July 1988 + + + Filter Samples + 1 2 4 8 16 + P[x=<X] 2756 2755 2755 2753 2749 + -------------------------------------------- + .1 12 9 8 7 6 + .2 24 19 16 14 14 + .3 36 27 22 20 19 + .4 51 36 29 25 23 + .5 69 47 36 30 27 + .6 93 61 44 35 32 + .7 133 80 56 43 35 + .8 215 112 75 53 43 + .9 589 199 111 76 63 + .99 1640 1002 604 729 315 + .999 5278 1524 884 815 815 + 1 10425 5325 991 835 815 + + Table D.5. Minimum Filter (UDEL - NCAR) + + Based on these data, the minimum filter was selected as the standard + algorithm. Since its performance did not seem to much improve for + values of n above eight, this value was chosen as the standard. + Network Time Protocol (Version 1): Specification and Implementation. + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Mills [Page 55] + +RFC 1059 Network Time Protocol July 1988 + + +Appendix E. NTP Synchronization Networks + + This section discusses net configuration issues for implementing a + ubiquitous NTP service in the Internet system. Section E.1 describes + the NTP primary service net now in operation, including an analysis + of failure scenarios. Section E.2 suggests how secondary service + nets, which obtain wholesale time from the primary service net, can + be configured to deliver accurate and reliable retail time to the + general host population. + +E.1. Primary Service Network + + The primary service net consists of five primary servers, each of + which is synchronized via radio or satellite to a national time + standard and thus operates at stratum one. Each server consists of + an LSI-11 Fuzzball, a WWVB or GOES radio clock and one or more net + interfaces. Some servers provide switching and gateway services as + well. Table E.1 shows the name, Internet address, type of clock, + operating institution and identifying code. + +Name Address Clock Operating Institution and (Code) +---------------------------------------------------------------------- +DCN5.ARPA 128.4.0.5 WWVB U Delaware, Newark, DE (UDEL) +FORD1.ARPA 128.5.0.1 GOES Ford Research, Dearborn, MI + (FORD) +NCAR.NSF.NET 128.116.64.3 WWVB National Center for Atmospheric + Research, Boulder, CO (NCAR) +UMD1.UMD.EDU 128.8.10.1 WWVB U Maryland, College Park, MD + (UMD) +WWVB.ISI.EDU 128.9.2.129 WWVB USC Information Sciences + Institute, Marina del Rey, CA + (ISI) + + Table E.1. Primary Servers + + Figure E.1 shows how the five primary servers are interconnected as + NTP peers. Note that each server actively probes two other servers + (along the direction of the arrows), which means these probes will + continue even if one or both of the two probed servers are down. On + the other hand, each server is probed by two other servers, so that + the result, assuming all servers are up, is that every server peers + with every other server. + + + + + + + + + +Mills [Page 56] + +RFC 1059 Network Time Protocol July 1988 + + + +------------------------------------------------+ + V | + +--------+ +--------+ +--------+ + | |<-------------| |<-------------| | + | NCAR | | ISI | | FORD | + | |----+ +--| |<--+ +---->| | + +--------+ | | +--------+ | | +--------+ + | | | | | A + | +---|------|---------------|----+ | + | | | | | | + | | +------|---------------|---------+ | + | | | | | | + | | | | V | + | +--------+ | | +--------+ | + | | |<--+ +--| | | + +-->| UMD | | UDEL |---+ + | |--------------------->| | + +--------+ +--------+ + + Figure E.1. Primary Service Network + + All of the five primary servers shown are directly connected to a + radio clock and thus normally operate at stratum one. However, if + the radio clock itself becomes disabled or the propagation path to + its synchronizing source fails, then the server drops to stratum two + and synchronizes via NTP with its neighbor at the smallest + synchronizing distance. If a radio clock appears to operate + correctly but delivers incorrect time (falseticker), the server may + remain synchronized to the clock. However, gross discrepancies will + become apparent via the NTP peer paths, which will ordinarily result + in an operator alarm. + + Assume that, if a radio clock appears up, it is a truechimer; + otherwise, the clock appears down. Then the above configuration will + continue to provide correct time at all primary servers as long as at + least one radio clock is up, all servers are up and the servers + remain connected to each other through the net. The fact that the + graph and all of its subgraphs are completely connected lends an + incredible resilience to the configuration. + + If some radio clocks appear up but are in fact falsetickers, the + primary servers connected to those clocks will not provide correct + time. However, as the consequents of the voting procedure and + complete connectivity of the graph and its subgraphs, any combination + of two falsetickers or of one falseticker and one down server will be + detected by their truechimer neighbors. + + + + + +Mills [Page 57] + +RFC 1059 Network Time Protocol July 1988 + + +E.2. Secondary Service Networks + + A secondary server operating at stratum n > 1 ordinarily obtains + synchronization using at least three peer paths, two with servers at + stratum n-1 and one or more with servers at stratum n. In the most + robust configurations a set of servers agree to provide backup + service for each other, so distribute some of their peer paths over + stratum-(n-1) servers and others over stratum-n servers in the same + set. For instance, in the case of a stratum-2 service net with two + secondary servers and the primary service net of Figure E.1, there + are five possible configurations where each stratum-1 path ends on a + different primary server. Such configurations can survive the loss + of three out of the four stratum-1 servers or net paths and will + reject a single falseticker on one of the two stratum-1 paths for + each server. + + Ordinary hosts can obtain retail time from primary or secondary + service net using NTP in client/server mode, which does not require + dedicated server resources as does symmetric mode. It is anticipated + that ordinary hosts will be quite close to a secondary server, + perhaps on the same cable or local net, so that the frequency of NTP + request messages need only be high enough, perhaps one per hour or + two, to trim the drift from the local clock. + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Mills [Page 58] + |